A spherosilicate oligomer with eight stable silanol groups as a building block of hybrid materials

Abstract

Diphenylsilylated double four-membered ring (D4R) spherosilicate possessing eight stable silanol groups ((R¹R²(HO)SiO)₈Si₈O₁₂, R¹ and R² = Ph: PP(OH)-D4R) has been synthesized by silylation of D4R silicate ([Si₈O₁₂]⁸⁻) with chloroethoxydiphenylsilane (ClSi(OEt)Ph₂) followed by subsequent hydrolysis. PP(OH)-D4R solid is air stable for at least 1 week. Ethoxymethylphenylsilylated and ethoxydimethylsilylated D4R oligomers (R¹R²(EtO)SiO)₈Si₈O₁₂, R¹ = Me and R² = Ph abbreviated as MP(OEt)-D4R and R¹ and R² = Me as MM(OEt)-D4R) do not provide spherosilicates possessing stable silanol groups and eventually gel (MP(OEt)-D4R-G and MM(OEt)-D4R-G, respectively) as hydrolyzed solutions dry. This suggests that bulky phenyl groups effectively stabilize terminal silanols. Although the PP(OH)-D4R silanols are stable, they can be trimethylsilylated or condensed to form Si–O–Si bonds by thermal-treatment. PP(OH)-D4R on a glass substrate cross-links upon heating. The structural periodicity and distance between D4R units in the heat-treated sample are higher and longer than those of MP(OEt)-D4R-G and MM(OEt)-D4R-G due to the steric effects of the bulky Ph groups. Additionally, the silica-based gel derived from PP(OH)-D4R also possesses a higher molecularly ordered structure than that derived from a mixture of Si(OEt)₄ and Ph₂Si(OEt)₂. This means that a building block approach is efficient for preparing molecularly ordered hybrid materials. The new spherosilicate possessing silanol groups will be a useful building block for preparation of various materials such as metallosilicates and mesostructured materials.